Homework Help: Bending stress in gear/bearing puller

1. Aug 9, 2011

gomerpyle

1. The problem statement, all variables and given/known data

I have a project where I have to determine the bending stress on a gear/bearing puller. It looks like this: http://www.posilock.com/PDFs/106.pdf [Broken].

The free body diagram I'm pretty sure would look something like this:

http://img20.imageshack.us/img20/1918/fbdi.jpg [Broken]

Basically these things grab onto gears or bearings, and a center bolt attached to a T-Handle is turned until it presses into the shaft on which the gear/bearing is mounted. So the jaws grabbing the gear/bearing pull one way while the center bolt pushes the other.

Now, these jaws have a maximum spread and a minimum spread. Apparently, when at minimum spread (jaws almost closed) they tend to break at point A (red dot at the tip). When at a maximum spread (jaws fully opened) they break elsewhere, point B (black dot). This does not make sense to me. Since bending stress is σ = Mc/I, wouldn't the puller always be breaking at the tip no matter how open or closed the jaws are? The reaction force Fx at the pin joint would be affecting both A and B about the same, but wouldn't the tip have a much smaller stiffness "I?" I guess my question would also be, how would you approximate the stiffness at point A. At point B the cross-sectional area is just a rectangle; simple.

I know the FBD does not have values or dimensions, but I am looking for a conceptual answer to this question. Also, the FBD is kind of rough. Realize that the tip protrudes off a bit more and is angled differently than in the crappy MS paint picture. See the pdf link for a more detailed drawing.

2. Relevant equations

Bending stress = Mc/I
I = bh^3/12

3. The attempt at a solution

When thinking about this, I sort of understand why the tip would break at a closed position. It seems like point B would be in the line of action of the reaction force at the pin, therefore reducing the bending stress at that area, so that tip would pick up most of the bending moment. At an open position, the only way point B would break before the tip is if the tip had a greater stiffness. I'm just not sure how to find it and incorporate it into the bending stress equation because its kind of a weird geometry at that point.

Last edited by a moderator: May 5, 2017